When performing contour machining, etc., by wire cut electric discharge machining methods, after forming a start hole in the workpiece, the wire electrode is passed through the start hole, to thereby create a machining gap between the wire electrode and the workpiece. Electric discharges are produced across the machining gap by intermittently applying a voltage between the wire electrode and workpiece and a numerical controller or the like is used to import a relative movement between the wire electrode and the workpiece, whereby the desired contour is machined.
When machining by the wire cut electric discharge method, as the machining approaches the finishing point, the core cut out by the machining may vibrate and be displaced resulting in a short circuit between the wire electrode and workpiece, thus interrupting machining.
In order to prevent such short circuiting, before completion of wire cut machining, it is necessary to maintain the corepiece in an immobile state relative to the workpiece. However, when so maintaining the core, the position of the wire guide which guides the wire electrode may be displaced, especially in the case of taper cut machining, thereby adversely influencing machining precision. Therefore, when fixing the corepiece position, it is desirable to have the corepiece immobilizing device within the guide block which contains the wire guide.
Also, for the purpose of performing unmanned machining over long time periods, after completion of the first wire cut machining, it is desirable that after the corepiece is removed from the workpiece, the machine be able to perform a second cut. Also, when machining several products from one workpiece, it is desirable, after cutting the corepiece out of the workpiece, and transferring it to a prescribed position, to automatically restart machining to machine the next contour.
Several corepiece handling devices have been suggested to maintain the corepiece against the workpiece, or to transfer it to a prescribed position after it is cut out from the workpiece. For example, Jpn. A 61-109617 discloses that after fixing a ring-shape electromagnet around a nozzle device, the electromagnet may be excited, thereby attaching any corepiece composed of a magnetic material to the electromagnet. When the excitation is cut off, the corepiece will become detached from the electromagnet.
However, many different sizes of corepieces may be cut out by wire cut electroerosion machining and each requires an appropriately sized holding device. When an electromagnet holder of the type which is screwed onto the periphery of the guide block having the flushing nozzle thereon, is used to support a ring-shape electromagnet, which magnetically attracts the corepiece via a ball bearing, the electromagnet or the electromagnet holder should be replaced according to the size of the corepiece to be handled. Such replacement work is complicated. Also, the need to change the corepiece handling devices causes a decrease in the overall machining efficiency, and renders automatic operation impossible.
Also, in Jpn. A, 62-38095, there is disclosed a structure that attracts and fixes both the cutout part and the non-cutout part of the workpiece. The structure comprises several permanent magnets installed in a substrate which can be freely attached and removed from the tip of the machining head. Removal is accomplished by exciting an arbitrary, desired permanent magnet using a coil controlled by a controller. However, in this composition, the structure used for maintaining the corepiece fixed to the machining head is undesirably large.